Question 338081
{{{(3x+7y)(3x-7y)}}} Start with the given expression.



Now let's FOIL the expression.



Remember, when you FOIL an expression, you follow this procedure:



{{{(highlight(3x)+7y)(highlight(3x)-7y)}}} Multiply the <font color="red">F</font>irst terms:{{{(3*x)*(3*x)=9*x^2}}}.



{{{(highlight(3x)+7y)(3x+highlight(-7y))}}} Multiply the <font color="red">O</font>uter terms:{{{(3*x)*(-7*y)=-21*x*y}}}.



{{{(3x+highlight(7y))(highlight(3x)-7y)}}} Multiply the <font color="red">I</font>nner terms:{{{(7*y)*(3*x)=21*x*y}}}.



{{{(3x+highlight(7y))(3x+highlight(-7y))}}} Multiply the <font color="red">L</font>ast terms:{{{(7*y)*(-7*y)=-49*y^2}}}.



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So we have the terms: {{{9*x^2}}}, {{{-21*x*y}}}, {{{21*x*y}}}, {{{-49*y^2}}} 



{{{9*x^2-21*x*y+21*x*y-49*y^2}}} Now add every term listed above to make a single expression.



{{{9*x^2-49*y^2}}} Now combine like terms.



So {{{(3x+7y)(3x-7y)}}} FOILs to {{{9*x^2-49*y^2}}}.



In other words, {{{(3x+7y)(3x-7y)=9*x^2-49*y^2}}}.



So you are correct. The answer is C). Good job.



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